Examples of conventional hole drilling tools for machining a hole in a work material, such as metal, include a throwaway type drill. This drill detachably mounts, on the tip end portion of a substantially cylindrical holder, an inner blade insert provided with an inner cutting edge part for cutting the inner peripheral side of a hole bottom surface, and an outer blade insert provided with an outer cutting edge part for cutting the outer peripheral side of the hole bottom surface. On this occasion, the rotation loci of the inner cutting edge part and the outer cutting edge part intersect each other.
Here, there is an insert for a drill which is provided with both of the inner cutting edge part and the outer cutting edge part (for example, refer to Japanese Unexamined Patent Application Publication No. 11-235606). This type of insert for a drill (hereinafter also referred to simply as an “insert”) is convenient because it is applicable to both of the inner blade insert and the outer blade insert.
The insert as described in the above publication has a cutting edge at an intersection part between the top surface and the side surface of an insert main body in a polygonal plate shape. The cutting edge has an inner cutting edge part and an outer cutting edge part adjacent to the inner cutting edge part. A through hole extending to the bottom surface of the insert is formed at the central part of the top surface. The through hole is used for fixing the insert to an insert pocket of a holder, and the insert has a 180-degree rotationally symmetric shape with respect to the central axis of the through hole.
In the top surface, a breaking groove is formed in a concave shape along the cutting edge. The breaking groove is used for disposing of chips, and has a rake face part and a rise face part in order of increasing distance from the cutting edge. The rake angle of the rake face part and the rise angle of the rise face part are adjusted to any angle, respectively, depending on work materials.
In use, two inserts of this type are mounted on a holder. One of these two inserts is mounted as an inner blade insert and the other is mounted as an outer blade insert on the tip end part of the holder. Specifically, the inner blade insert and the outer blade insert are mounted on the tip end part of the substantially cylindrical shaped holder in different mounting directions, namely, radially inwardly of the holder and radially outwardly of the holder, respectively.
That is, the inner blade insert and the outer blade insert are mounted so that the inner cutting edge part and the outer cutting edge part are projected from axial tip end of the holder, respectively. On this occasion, the rotation loci of the inner cutting edge part and the outer cutting edge part on the axial tip end side intersect each other. Then, by rotating the holder around the axis of the holder, hole machining of a work material is carried out by the inner blade insert and the outer blade insert.
Here, each of the inner cutting edge part and the outer cutting edge part in the insert has a bent portion that is bent in a convex shape when the insert is viewed from above. The bent portion is used for adjusting the balance of cutting resistance exerted on the inner cutting edge part and the outer cutting edge part. However, when the bent portion is present, with the insert mounted on the holder, cutting edges having opposite inclinations with the bent portion in between are formed in the cutting edge parts with respect to the hole bottom surface.
That is, inner cutting edge parts having the opposite inclinations with the bent portion in between are formed in the inner cutting edge part of the inner blade insert with respect to the hole bottom surface. Outer cutting edge parts having the opposite inclinations with the bent portion in between are formed in the outer cutting edge part of the outer blade insert with respect to the hole bottom surface.
When hole machining is performed by using the above-mentioned insert, chips are generated from the opposite directions with the bent portion in between, and hence partially generated chips will collide with each other. Therefore, chips generated in the vicinity of the bent portion are considerably squeezed, and the chip thickness in the vicinity of the bent portion may be increased. As a result, there has been the problem that the chips are welded onto the wall surface of the breaking groove corresponding to such a portion, and this portion is subjected to local increase in cutting resistance, resulting in the cutting edge chipping. This problem has become significant in the outer cutting edge part whose rotation speed is high.